1. Field of the Invention
The present invention relates to a method and apparatus for establishing feedback channels in an MBMS (multimedia broadcast/multicast service) for UMTS (Universal Mobile Telecommunication System) and, more particularly, a method for selectively establishing an RRC connection with the terminals that are capable of transmitting uplink feedback information regarding the particular MBMS service.
2. Description of the Related Art
A universal mobile telecommunication system (UMTS) is a European-type, third generation IMT-2000 mobile communication system that has evolved from a European standard known as Global System for Mobile communications (GSM). UMTS is intended to provide an improved mobile communication service based upon a GSM core network and wideband code division multiple access (W-CDMA) wireless connection technology.
In December 1998, a Third Generation Partnership Project (3GPP) was formed by the ETSI of Europe, the ARIB/TTC of Japan, the T1 of the United States, and the TTA of Korea. The 3GPP creates detailed specifications of UMTS technology. In order to achieve rapid and efficient technical development of the UMTS, five technical specification groups (TSG) have been created within the 3GPP for standardizing the UMTS by considering the independent nature of the network elements and their operations.
Each TSG develops, approves, and manages the standard specification within a related region. Among these groups, the radio access network (RAN) group (TSG-RAN) develops the standards for the functions, requirements, and interface of the UMTS terrestrial radio access network (UTRAN), which is a new radio access network for supporting W-CDMA access technology in the UMTS.
FIG. 1 illustrates an exemplary basic structure of a general UMTS network. As shown in FIG. 1, the UMTS is roughly divided into a terminal or user equipment (UE) 10, a UTRAN 20, and a core network (CN) 30.
The UTRAN 20 includes one or more radio network sub-systems (RNS) 25. Each RNS 25 includes a radio network controller (RNC) 23 and a plurality of Node-Bs (base stations) 21 managed by the RNC 23. The RNC 23 handles the assignment and management of radio resources and operates as an access point with respect to the core network 30.
The Node-Bs 21 receive information sent by the physical layer of the terminal 10 through an uplink and transmit data to the terminal 10 through a downlink. The Node-Bs 21 operate as access points of the UTRAN 20 for the terminal 10.
The UTRAN 20 constructs and maintains a radio access bearer (RAB) for communication between the terminal 10 and the core network 30. The core network 30 requests end-to-end quality of service (QoS) requirements from the RAB, and the RAB supports the QoS requirements the core network 30 has set. Accordingly, by constructing and maintaining the RAB, the UTRAN 20 can satisfy the end-to-end QoS requirements.
The services provided to a specific terminal 10 are roughly divided into the circuit switched (CS) services and the packet switched (PS) services. For example, a general voice conversation service is a circuit switched service, while a Web browsing service via an Internet connection is classified as a packet switched (PS) service.
For supporting circuit switched services, the RNCs 23 are connected to the mobile switching center (MSC) 31 of the core network 30 and the MSC 31 is connected to the gateway mobile switching center (GMSC) 33 that manages the connection with other networks. For supporting packet switched services, the RNCs 23 are connected to the serving general packet radio service (GPRS) support node (SGSN) 35 and the gateway GPRS support node (GGSN) 37 of the core network 30. The SGSN 35 supports the packet communications with the RNCs 23 and the GGSN 37 manages the connection with other packet switched networks, such as the Internet.
FIG. 2 illustrates a structure of a radio interface protocol between the terminal 10 and the UTRAN 20 according to the 3GPP radio access network standards. As shown in FIG. 2, the radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane (U-plane) for transmitting user data and a control plane (C-plane) for transmitting control information.
The user plane is a region that handles traffic information with the user, such as voice or Internet protocol (IP) packets. The control plane is a region that handles control information for an interface with a network, maintenance and management of a call, and the like.
The protocol layers in FIG. 2 can be divided into a first layer (L1), a second layer (L2), and a third layer (L3) based on the three lower layers of an open system interconnection (OSI) standard model.
The first layer (L1), namely, the physical layer, provides an information transfer service to an upper layer by using various radio transmission techniques. The physical layer is connected to an upper layer called a medium access control (MAC) layer, via a transport channel. The MAC layer and the physical layer exchange data via the transport channel.
The second layer (L2) includes a MAC layer, a radio link control (RLC) layer, a broadcast/multicast control (BMC) layer, and a packet data convergence protocol (PDCP) layer.
The MAC layer handles mapping between logical channels and transport channels and provides allocation of the MAC parameters for allocation and re-allocation of radio resources. The MAC layer is connected to an upper layer called the radio link control (RLC) layer, via a logical channel.
Various logical channels are provided according to the type of information transmitted. In general, a control channel is used to transmit information of the control plane and a traffic channel is used to transmit information of the user plane.
A logical channel may be a common channel or a dedicated channel depending on whether the logical channel is shared. Logical channels include a dedicated traffic channel (DTCH), a dedicated control channel (DCCH), a common traffic channel (CTCH), a common control channel (CCCH), a broadcast control channel (BCCH), and a paging control channel (PCCH). The BCCH provides information including information utilized by a terminal 10 to access a system. The PCCH is used by the UTRAN 20 to access a terminal 10.
The MAC layer is connected to the physical layer by transport channels and can be divided into a MAC-b sub-layer, a MAC-d sub-layer, a MAC-c/sh sub-layer, and a MAC-hs sub-layer according to the type of transport channel being managed. The MAC-b sub-layer manages a BCH (Broadcast Channel), which is a transport channel handling the broadcasting of system information. The MAC-c/sh sub-layer manages a common transport channel, such as a forward access channel (FACH) or a downlink shared channel (DSCH), which is shared by a plurality of terminals. The MAC-d sub-layer manages a dedicated channel (DCH), which is a dedicated transport channel for a specific terminal 10. Accordingly, the MAC-d sublayer is located in a serving RNC (SRNC) that manages a corresponding terminal, and one MAC-d sublayer also exists in each terminal.
The RLC layer supports reliable data transmissions and performs segmentation and concatenation on a plurality of RLC service data units (SDUs) delivered from an upper layer. When the RLC layer receives the RLC SDUs from the upper layer, the RLC layer adjusts the size of each RLC SDU in an appropriate manner based upon processing capacity and then creates data units by adding header information thereto. The data units, called protocol data units (PDUs), are transferred to the MAC layer via a logical channel. The RLC layer includes a RLC buffer for storing the RLC SDUs and/or the RLC PDUs.
The BMC layer schedules a cell broadcast (CB) message transferred from the core network and broadcasts the CB message to terminals 10 positioned in a specific cell or cells.
The PDCP layer is located above the RLC layer. The PDCP layer is used to transmit network protocol data, such as the IPv4 or IPv6, effectively on a radio interface with a relatively small bandwidth. For this purpose, the PDCP layer reduces unnecessary control information used in a wired network, a function called header compression.
The radio resource control (RRC) layer located at the lowest portion of the third layer (L3) is only defined in the control plane. The RRC layer controls the transport channels and the physical channels in relation to setup, reconfiguration, and the release or cancellation of the radio bearers (RBs). The RB signifies a service provided by the second layer (L2) for data transmission between the terminal 10 and the UTRAN 20. In general, the set up of the RB refers to the process of stipulating the characteristics of a protocol layer and a channel required for providing a specific data service, and setting the respective detailed parameters and operation methods.
The RRC state refers to whether there exists a logical connection between the RRC of the terminal 10 and the RRC of the UTRAN 20. If there is a connection, the terminal 10 is said to be in RRC connected state. If there is no connection, the terminal 10 is said to be in idle state.
Because an RRC connection exists for terminals 10 in RRC connected state, the UTRAN 20 can determine the existence of a particular terminal within the unit of cells, for example which cell the RRC connected state terminal is in. Thus, the terminal 10 can be effectively controlled.
In contrast, the UTRAN 20 cannot determine the existence of a terminal 10 in idle state. The existence of idle state terminals 10 can only be determined by the core network 30 to be within a region that is larger than a cell, for example a location or a routing area. Therefore, the existence of idle state terminals 10 is determined within large regions, and, in order to receive mobile communication services such as voice or data, the idle state terminal must move or change into the RRC connected state.
The 3GPP system can provide multimedia broadcast multicast service (MBMS), which is a new type of service in Release 6. The 3GPP TSG SA (Service and System Aspect) defines various network elements and their functions required for supporting MBMS services. A cell broadcast service provided by the conventional Release 99 is limited to a service in which text type short messages are broadcast to a certain area. The MBMS service provided by Release 6 is a more advanced service that multicasts multimedia data to terminals (UEs) 10 that have subscribed to the corresponding service in addition to broadcasting multimedia data.
The MBMS service is a downward-dedicated service that provides a streaming or background service to a plurality of terminals 10 by using a common or dedicated downward channel. The MBMS service is divided into a broadcast mode and a multicast mode.
The MBMS broadcast mode facilitates transmitting multimedia data to every user located in a broadcast area, whereas the MBMS multicast mode facilitates transmitting multimedia data to a specific user group located in a multicast area. The broadcast area signifies a broadcast service available area and the multicast area signifies a multicast service available area.
FIG. 3 illustrates a process of providing a particular MBMS service (service 1) by using multicast mode. Users who desire to receive the MBMS service, for example UE1 and UE2, first receive a service announcement provided by a network. The service announcement provides the terminal 10 with a list of services to be provided and related information. In addition, the users must receive a service notification provided by the network. The service notification provides the terminal 10 with information related to the broadcast data to be transmitted.
If the user intends to receive the multicast mode MBMS service, the user subscribes to a multicast subscription group. A multicast subscription group is a group of users who have completed a subscription procedure. Once a user has subscribed to the multicast subscription group, the user can join a multicast group to receive a specific multicast service. A multicast group is a group of users that receive a specific multicast service. Joining a multicast group, also referred to as MBMS multicast activation, involves merging with the multicast group that has users who wish to receive the specific multicast service. Accordingly, the user can receive the specific multicast data by joining a multicast group, referred to as MBMS multicast activation. Each terminal 10 may individually subscribe to a multicast subscription group and join or leave a multicast group before, during, or any time after data transmission.
While a particular MBMS service is in progress, one or more sessions for that service may occur in sequence. When data to be transmitted for a particular MBMS service is generated at the MBMS data source, the core network 30 indicates the start of a session to the RNC 23. In contrast, when there is no further data to be transmitted for a particular MBMS service, the core network 30 indicates a session stop to the RNC 23.
Between session start and session stop, data transmission for the particular MBMS service is performed. Only those terminals 10 that have joined a multicast group for the MBMS service may receive data that during the data transmission.
In the session start procedure, the UTRAN 20 that received the session start from the core network 30 transmits an MBMS notification to the terminals 10. MBMS notification involves to UTRAN 20 informing the terminal 10 that transmission of data for a particular MBMS service within a certain cell is impending.
The UTRAN 20 can use the MBMS notification procedure to perform a counting operation that determines the number of terminals 10 that wish to receive a particular MBMS service within a particular cell. The counting procedure is used to determine whether the radio bearer for providing the particular MBMS service should be set as point-to-multipoint or point-to-point.
To select the MBMS radio bearer, the UTRAN 20 internally establishes a threshold value. After performing the counting function, the UTRAN 20 may set a point-to-point MBMS radio bearer if the number of terminals 10 existing within the corresponding cell is smaller than a threshold value and may set a point-to-multipoint MBMS radio bearer if the number of terminals existing within the corresponding cell is greater than or equal to the threshold value.
When a point-to-point radio bearer is set for a particular service, the terminals 10 wishing to receive the corresponding service are all in an RRC connected state. However, when a point-to-multipoint radio bearer is set for a particular service, all the terminals 10 wishing to receive the corresponding service need not be in an RRC connected state since terminals in an idle state may also receive the point-to-multipoint radio bearer.
The MBMS service transmitting multimedia data by broadcast or multicast may employ RTP (Real-time Transport Protocol) for transmitting packets in real-time. RTP is a protocol made appropriately for transmitting data having real-time characteristics, such as audio or video, over a multicast network or a unicast network. When using RTP for transmitting data in real-time on the downlink, an RTCP (RTP Control Protocol) that controls the downlink transmission of real-time data can be used for the uplink.
In MBMS, uplink RTCP packet transmissions are performed through a point-to-point radio bearer, which is different from the MBMS radio bearer used in transmitting downlink RTP packet transmissions. The main function of RTCP is to provide feedback of condition information regarding data allocation that is related to flow control and congestion control of other protocols. More specifically, the RTCP packets indicate an amount of RTP packets that are lost during transmission from the source to the final destination. This information is used for controlling the size of the RTP packets and for finding an appropriate encoding method.
In the related art, when a point-to-multipoint radio bearer is set for a particular service within a particular cell, the UTRAN 20 may allow some terminals 10 to stay in RRC connected state based upon radio resource managing conditions while requiring the remaining terminals to be in idle state. For example, when the UTRAN 20 receives an RRC connection request message from the terminals 10 wishing to receive a particular service, RRC connection setup messages are sent to a limited number of terminals according to the radio resource managing conditions for controlling the reception of the corresponding service in RRC connected state. RRC connection reject messages are transmitted to the other terminals 10 so that these terminals may receive the corresponding service in idle state.
FIG. 4 illustrates a signal flow chart indicating successful RRC connection setup according to the conventional art. After an MBMS session start message is received from the core network 30 in step S50, the UTRAN 20 transmits an MBMS notification message to those terminals 10 wishing to receive the corresponding MBMS service in step S52 to indicate that data transmission for the particular MBMS service is impending.
Each terminal 10 that receives the MBMS notification message transmits an RRC connection request message to the UTRAN 20 in step S54. The UTRAN 20 considers the current condition of radio resources and determines, in step S56, which RRC connections should be granted with a limited number of terminals 10 below a threshold value.
In selecting which terminals 10 to grant an RRC connection, the UTRAN 20 randomly or sequentially selects terminals wishing to receive the MBMS service, for example on a first-come-first-served basis. The UTRAN 20 adjusts the total number of terminals 10 granted RRC connections to be at the threshold value and then establishes RRC connections.
In step S58 the UTRAN 20 transmits RRC connection setup messages to the terminals selected for RRC connection. The terminals that receive the RRC connection setup message then transmit an RRC connection setup complete message to the UTRAN 20 in step S60. Upon successfully completing this procedure, an RRC connection exists between each selected terminal 10 and the UTRAN 20 and each selected terminal is in RRC connected state. In step S62, the UTRAN allows those terminals not selected for RRC connection that desire to receive the MBMS service to set up a point-to-multipoint MBMS bearer.
FIG. 5 illustrates a signal flow diagram showing unsuccessful RRC connection setup according to the conventional art. After an MBMS session start message is received from the core network 30 in step S50, the UTRAN 20 transmits an MBMS notification message to those terminals 10 wishing to receive the corresponding MBMS service in step S52 to indicate that data transmission for the particular MBMS service is impending.
Each terminal 10 that receives the MBMS notification transmits an RRC connection request message to the UTRAN 20 in step S54. The UTRAN 20 considers the current condition of radio resources and determines, in step S56, which RRC connections should be granted with a limited number of terminals 10 below a threshold value.
As illustrated in FIG. 5, the UTRAN 20 determines that RRC connections should not be granted for those terminals 10 that exceed the threshold value, for example any terminal counted after the number of terminals reached the threshold value. The UTRAN 20 transmits, in step S59, RRC connection reject messages to those terminals determined not to require an RRC connection. The terminals receiving an RRC connection reject message are in idle state. In step S62, the UTRAN allows those terminals not selected for RRC connection that desire to receive the MBMS service to set up a point-to-multipoint MBMS bearer.
As illustrated in FIGS. 4 and 5, when a point-to-multipoint radio bearer is provided for a particular service within a particular cell, some terminals 10 desiring to receive the service are in RRC connected state, while the remaining terminals are in idle state. If feedback information is required for the MBMS service and the MBMS service is provided on the downlink using RTP and is received on the uplink using RTCP, only those terminals in RRC connected state may transmit feedback information because transmission of feedback information employs a point-to-point radio bearer and only terminals in RRC connected state may establish a point-to-point radio bearer.
Furthermore, in order to transmit feedback information, such as RTCP, while receiving MBMS service, the terminal 10 must be able to simultaneously receive both the point-to-multipoint radio bearer and the point-to-point radio bearer. Therefore, even a terminal 10 in RRC connected state cannot transmit feedback information regarding the MBMS service if the terminal cannot simultaneously receive both a point-to-multipoint radio bearer and a point-to-point radio bearer.
In general, for MBMS service, the radio network should control the downlink MBMS data transmission using uplink feedback information from a plurality of terminals 10. For downlink MBMS data transmission, it is desirable that feedback information be received from as many terminals 10 as possible because the conditions for all terminals must be considered.
Only those terminals 10 that are in RRC connected state and can simultaneously receive both a point-to-multipoint and a point-to-point radio bearer may transmit uplink feedback information. Therefore, considering the limited amount of radio resources, it is preferable that the radio network first establishes an RRC connection with those terminals 10 that are able to simultaneously receive both a point-to-multipoint and a point-to-point radio bearer. Furthermore, considering that the feedback information of one or more terminals 10 effect MBMS data transmissions, the radio network should give priority to the terminals for “high-end” users, for example users with expensive mobile phones with feedback functions, when determining which terminals are granted RRC connections so that feedback information transmitted from the terminals of such “high-end” users is always considered when providing MBMS services to various users.
However, prior to sending an RRC connection setup message, the radio access network, for example a UTRAN 20 of the conventional art, cannot tell whether a terminal 10 is able to simultaneously receive both a point-to-multipoint and a point-to-point radio bearer. In the conventional art, the UTRAN 20 randomly or sequentially selects the terminals to receive RRC connections from the terminals that have sent RRC connection request messages, for example on a first-come-first-served basis. Therefore, feedback capability is not considered when selecting the terminals to receive RRC connections in the conventional art.
Therefore, there is a need for a method and apparatus that facilitates the consideration of a terminal's ability to provide feedback when determining which terminals will receive RRC connections. The present invention addresses this and other needs.